I don't know how helpful this is, but personally I wouldn't want another car off a trader I don't trust- even if the replacement car isn't technically going to cost anything up front.
And how do you really feel about the grey car? Do you actually want it, or would you just be taking it because it's the easier option? I only ask because I was quite specific about the spec and colour of Supra that I wanted when I was shopping around, and I don't think I'd have been happy if I'd settled for something that wasn't what I was after.
Hope things get sorted either way, and I hope this doesn't put you off Supra ownership! I think you should keep pushing for the refund.